Regular locking device generally utilizes a latch bolt having a rectilinear motion, or a hook bolt having a circular motion so as to lock or unlock a door. The former tends to be damaged or deformed while the latter is more difficult to drive in the notch of an associated striking plate. The hook bolt may produce noise during its operation.
The lock which locks a door by means of a rectilinear motion of the latch bolt may be easily destroyed by a burglar through violence. In a regular one-step locking device, the single latch bolt is driven to insert only a short distance into the notch of a striking plate mounted on a door frame. Through severe impact force against the door, the door panel and the latch bolt will be easily deformed, and the latch bolt will be pushed out of the associated striking plate to unlock the door (This is commonly seen in TV programs). In regular two or three-step locking device (which comprises two or three latch bolts), the door still cannot be well protected against violence. Although the latch bolts are inserted more deeply in the notches of a striking plate, the motion range of the latch bolts into the notches of a striking plate is still limited by the thickness of a door frame. If a regular two or three-step locking device is used to lock a door, a burglar may hit the door, open it after having inserted a crowbar into the gap between the door panel and the associated door frame (in order to facilitate the turning of a door panel against a door frame, a gap and possible tolerance must be remained therebetween) to pry the door panel in or out to a certain extent.